ArtSkills is a website where you and your kids can make posters. You need to register - a simple process. Then you can check out posters others have created in the Poster Gallery, or make your own.
The poster wizard offers you a range of templates to help you get started. They are customisable, so you can add text, images, change colours and fonts etc. I needed an undo button when I tried a text alignment that looked poor with the number of letters I chose. I couldn't find one, so I had to start over, but that didn't take long. When finished, you can save your poster, and the site will email you links for further action on it eg a printable version, and a version that is best for you to use to reconstruct your poster by hand.
emergency poster guide for parents - "Mum, I just remembered, I have to make a poster for tomorrow!" The advice is for real life posters, as well as digital ones.
Art Skills goes on my list of websites that encourage children's literacy. It's also a good resource to remember when you need to customise and print a poster that advertises your garage sale or meeting. Find other poster making resources in Book Chook Favourites - Making Posters.